This invention is particularly concerned with deciding at a receiver which of a predetermined set of symbols was transmitted, by using an error correction or detection decoder operating on a recovered signal having a number of possible values greater than the number of symbols in the set--called soft decision decoding. More particularly, in a communications network having time-dispersed signals, this invention is concerned with soft decision decoding using information derived during equalization.
In digital transmission over a medium vulnerable to corruption, such as a fading radio channel, convolutional error correction coding is used to distribute transmitted information over time such that it can still be reconstructed, despite some corruption. An error correction decoding technique, such as Viterbi algorithm channel decoding, is used to reconstruct the originally-transmitted information from the corrupted signal.
In a digital radiotelephone system, high data rate digital radio transmissions encounter serious intersymbol interference (ISI) due to multiple reflections and echoes off buildings and other objects in the transmission path. The ISI is especially severe in severely delay-spread channels where the various reflections arrive at the receiver delayed by amounts comparable to a significant fraction of a symbol time. Thus, adaptive equalization--attempting to account for these multipath, time-dispersal effects on the signal and realign the echoes in time--is employed prior to error correction or detection. The problem is that equalization attempts to distill the information carried in the composite, multipath signal to a definitive decision about each symbol received--a "hard decision". For example, in the binary case, where one of only two symbols can be sent, the output of the equalizer can only take on one of two values, these values being referred to as the equalizer's hard decisions.
It is well recognized that better decisions can be made about the value of any individually-transmitted symbol with the availability of "soft information". That is, if during error correction, it were known to the decoder what the quality of the signal was when the symbol was received, a better decision could ultimately be made about what the true value of the transmitted symbol really was. This soft information represents a measure of the confidence held in any given symbol detection. But any such soft information has ordinarily been removed by equalization.
Conventional wisdom dictates that soft information be developed at or ahead of the radio demodulator (at the RF, IF or discriminator stage), at a point that is rich in signal quality information (signal strength, noise and distortion). See, for example, FIG. 6 of Tadashi Matsumoto, "Soft Decision Decoding of Block Codes Using Received Signal Envelope in Digital Mobile Radio", IEEE Journal on Selected Areas in Comm., Vol. 7, No. 1, January 1989, or "Modulation and Channel Coding in Digital Mobile Radio Telephony", Proceedings of the Nordic Seminar on Digital Land Mobile Radiocommunication, Espoo, Finland, Feb. 5-7, 1985, pp. 219-227. Yet it is not widely recognized that the composite signal envelope yields erroneous information in the severely delay-spread channel. This may possibly be due to the phenomenon of individual signal paths causing rapid signal envelope changes (hereinafter coined "microfading"), changes taking place much more rapidly than is known for conventional Rayleigh fading at a given receiver speed.
This invention takes as its object to overcome these shortcomings. In the copending application referenced above, we recognized that performance superior to that achieved by using the output of the matched filter might be achieved by using the MLSE branch metrics. This invention also takes as its object to realize that advantage and further exploit the operating features of the MLSE.